Current running times are 43-44 minutes, so no way you can run a half hourly service with three units. Is there any potential for skip-stop calling patterns for some of the lessor used stations? An hourly all shacks and an hourly semi-fast taking less than 40mins would be just about do-able with 3 units, but tight if anything goes wrong.
Unit 1: Bed 0700>Bly 0743 (stopper), Bly 0748>Bed 0825 (fast) > 0830, etc
Unit 2: Bed 0730>Bly 0808 (fast), Bly 0812>Bed 0855 (stopper) > 0900, etc
Unit 3: Bed 0800>Bly 0843 (stopper), Bly 0848>Bed 0925 (fast) > 0930, etc
Perhaps up to 1000 with half hourly, hourly to 1600, then half hourly until 1900 and hourly again thereafter?
Just a thought...